This invention relates to a method of making a reel-pressure spring for magnetic tape cassettes or cartridges, such as the type commonly used in video cassette recorders. The invention is particularly concerned with a method of making a reel-pressure spring having a uniform width throughout, i.e., the spring has parallel edges. Such a spring is described and claimed in U.S. patent application Ser. No. 030,698, filed Mar. 25, 1987, now U.S. Pat. No. 4,770,367 and assigned to the present assignee. The disclosure of this application is incorporated herein by reference.
Reel springs are used in a cassette to rotatably mount the tape reel shafts. Prior art reel springs are shown in Maehara, U.S. Pat. No. 4,544,062, and Sato et al, U.S. Pat. No. 4,593,868. These springs have a generally rhombic outline, typically stamped out of a continuous strip of spring material. With a rhombic spring outline a significant portion of the metal strip is trimmed away and discarded. Another difficulty with a rhombic outline is the high number of lineal inches of material which must be stamped or cut. The high number of lineal inches of cut requires the use of a large, progressive die which breaks down the high total inches cut into small portions cut at a series of work stations.
FIG. 1 illustrates a representative sample of this prior art procedure for forming reel springs. FIG. 1 illustrates the strip material as it would look at the last two stations of an eleven station die. Springs 10 are formed in a carrier strip 12 of spring material. In this illustration two springs are made at each work station. The springs are nested as shown to utilize a greater percentage of the overall strip material. Spring 10A is complete, except for blanking through along the center tab lines 14. Once this is done the spring is ejected from the die into a bin for further processing. The ejected spring leaves an opening in the strip material, as shown at the work station to the right of spring 10A. This illustrates the high number of lineal inches of material that must be cut. The entire periphery 16 must be cut at one point or another in the progressive die.
This large amount of lineal inches of cut necessitates the use of a die lubricant. Some of the lubricant inevitably winds up on the finished springs, rendering them unfit for use until they are cleaned to remove the lubricant. This is typically done by tumbling the finished springs in corncob chips. After cleaning, the springs have to be oriented and stacked for packing. Since the springs will be stacked at random with respect to the order in which they were stamped out of the strip material, the tolerance variations between adjacent springs in the stack will similarly be entirely random. That is, one spring may be at the full positive tolerance allowed while an adjacent spring is at the full negative allowable tolerance. Both parts are within the specifications, but they are at opposite extremes of the tolerance envelope. This creates difficulties for the automated assembly equipment used to place the individual springs in a cassette. Such equipment can handle spring size variations within the tolerance envelope so long as the tolerance variations from one spring to the next are reasonably smooth or continuous. The assembly equipment, however, tends to falter when presented with abrupt changes of springs from one side of the tolerance envelope to the other. In the past, abrupt changes in the tolerance variation of adjacent springs in a stack have been handled by visually checking the stack and manually removing those parts which appear to be appreciably different in size from their neighbors. This process is slow, unreliable and labor intensive.
Another drawback with the prior art springs and the process for making them is the large die required is complex and expensive to make, and it requires a great deal of maintenance to keep it operating. The down time for die maintenance becomes a significant factor limiting the production available from the manufacturing process. In other words, the multiple station dies require a relatively high capital investment which is then tied up in non-productive time for die maintenance.
Yet another difficulty with the prior art is the type of coil used limits the supply of strip material. The multiple station die is most productive when making two parts at once, as shown in FIG. 1. This requires the use of a wide strip of spring material which can only be ribbon-wound on a coil. In a typical operation the ribbon-wound coil can supply a machine for about 45 minutes before a new one has to be installed, a procedure which takes about 15 minutes.